The present invention relates to Co-coated acicular magnetite particles and a process for producing the same, and more particularly, the present invention relates to Co-coated acicular magnetite particles; which have a high coercive force, a large saturation magnetization, an excellent black chromaticity and an excellent chemical and magnetic stability with passage of time (chemical and magnetic temporal stability), and hence, which are suitable as magnetic oxide particles for high-density recording media, and to a process for producing the same.
With the recent development of miniaturized and lightweight magnetic recording apparatuses, the necessity for a higher performance of a recording medium such as a magnetic tape and a magnetic disk has been increasing more and more.
In other words, a magnetic recording medium is required to have a high recording density, a small light transmittance, and an excellent magnetic and chemical stability.
The characteristics of the magnetic iron oxide particles which are demanded in order to satisfy the above-mentioned requirements to the magnetic recording medium, are to be high in coercive force, large in saturation magnetization, excellent in black chromaticity and excellent in chemical and magnetic stability with passage of time.
This fact is described in, for example, Japanese Patent Publication (KOKOKU) No. 55-6580 (1980), "There is an increasing tendency of a recording signal moving to a shorter-wavelength region, and this tendency is especially remarkable in video cassettes. That is, a magnetic recording medium is required to have a high recording density, high output characteristics, especially, improved frequency characteristics, and in addition, a magnetic stability. In order to meet such demands, a magnetic material for such a magnetic recording medium should have a high magnetic stability and a high coercive force (Hc)".
As magnetite particles having a high coercive force, what is called Co-doped magnetic iron oxide particles and what is called Co-coated magnetic iron oxide particles are conventionally known.
As is known, the Co-doped magnetic iron oxide particles are very unstable both chemically and magnetically, while the chemical and magnetic stability of the Co-coated magnetic iron oxide particles is rather excellent as compared with that of the Co-doped magnetic iron oxide particles. Especially, when the precursor particles are magnetite particles, Co-coated magnetic iron oxide particles have a higher coercive force, a larger saturation magnetization and a more excellent black chromaticity than those having maghemite particles as the precursor particles, so that the magnetite particles are expected to be used as magnetic iron oxide particles for high-density recording media.
With respect to black chromaticity, it is known that black chromaticity mainly depends on the Fe.sup.2+ content and that the black chromaticity has a tendency of increasing with an increase in the Fe.sup.2+ content, as described in pp. 239 to 240 of Powder and Powder Metallurgy, Vol. 26, No. 7, "The black chromaticity of the sample depends upon the Fe(II) content and the average particle diameter. The particles having an average particle diameter of 0.2 .mu.m are a bluish black powder which is the host suitable for a black pigment . . . When the Fe(II) content is not less than 10%, each sample assumes a black color, although there is a slight difference in the black chromaticity. If the Fe(II) content reduces to less than 10%, the black color of each sample changes to a reddish brown color". Since Co-coated magnetite particles contain Fe.sup.2+, they have a higher black chromaticity than Co-coated maghemite particles.
It is essential for magnetic recording media such as magnetic tapes, especially, video tapes to reduce the light transmittance by adding carbon black in a magnetic layer. However, addition of carbon black has the following problems. Addition of a large amount of carbon black which does not contribute to an increase in the magnetic properties obstructs the enhancement of the properties of the magnetic recording media. Furthermore, the existence of carbon black in a film composition, when it is produced by kneading magnetic iron oxide particles with a vehicle, obstructs the dispersion of the magnetic iron oxide particles in the vehicle, resulting in the lowering of the orientation and the packing property of the magnetic iron oxide particles in the coating film.
Since carbon black has a high bulk density of about 0.1 g/cm.sup.3, it is difficult to handle and the operability is not good. In addition, a safety and sanitary problem such as carcinogenesis is pointed out.
An attempt has been made of using Co-coated magnetite particles having a higher black chromaticity than conventional Co-coated maghemite particles in order to lessen the light transmittance of a video tape or the like, thereby reducing the carbon black content.
However, although Co-coated magnetite particles have more excellent properties than Co-coated maghemite particles, as described above, it is known Co-coated magnetite particles are magnetically and chemically unstable due to the existence of Fe.sup.2+ in the particles.
This fact is described in Japanese Patent Publication (KOKOKU) No. 55-6580 (1980), "Co-containing acicular magnetite particles have a high coercive force and a large saturation magnetization, . . . and they are expected to be used as a material for magnetic recording, but they are disadvantageous in that the coercive force largely changes with passage of time due to the existence of Fe.sup.2+ in the particles", and ". . . when Co-containing acicular magnetite particles are taken out in the air, they are oxidized and the Fe.sup.2+ content is reduced from the stoichiometric quantity, which may cause vacancies in a crystal lattice. When such Co-containing acicular magnetite particles are permitted to stand at room temperature, the positive ions (Fe.sup.2+, Co.sup.2+) by way of the vacancies move to a stable position, so that the coercive force is considered to gradually increase with passage of time . . . It is due to the coordination of ions in a crystal lattice that the coercive force changes with passage of time when Co-containing acicular magnetite particles are permitted to stand at room temperature . . . When the Co-containing acicular magnetite particles are permitted to stand at room temperature, the Fe.sup.2+, Co.sup.2+ ions by way of the vacancies move to a stable position, and as a result the coercive force changes with passage of time".
When Co-coated acicular magnetite particles are permitted to stand for a predetermined period, phenomena such as a reduction of Fe.sup.2+ content with passage of time (chemical instability) and a lowering of the saturation magnetization with passage of time (magnetic instability) are produced.
Various attempts of adding a magnesium salt have conventionally been made in order to improve various properties of Co-coated acicular iron oxide particles. For example, there are a method of adding an aqueous solution of a magnesium salt simultaneously with the addition of an aqueous solution of a ferrous salt or an aqueous solution of a cobalt salt, or between the addition of an aqueous solution of a ferrous salt and the addition of an aqueous solution of a cobalt salt in a cobalt coating process (Japanese Patent Publication (KOKOKU) No. 4-25686 (1992)); a method of dissolving a magnesium salt in a water dispersion of Co-coated ferromagnetic particles and adding an alkali hydroxide so as to deposit the magnesium hydroxide produced on the surfaces of ferromagnetic particles (Japanese Patent Publication (KOKOKU) No. 62-50889 (1987)); a method of repulping a cake obtained by filtering and washing Co-coated acicular iron oxide particles with water so as to slurry the particles again, and coating the surfaces of the particles with a hydroxide of magnesium by adding an aqueous solution of a magnesium salt to the slurry (Japanese Patent Publication (KOKOKU) No. 2-30563 (1990)); a method of adding a magnesium salt to washing water when the alkaline slurry of the Co-modified .gamma.-iron oxide particles obtained by modifying .gamma.-iron oxide particles by cobalt is washed with water (Japanese Patent Application Laid-Open (KOKAI) No. 1-184801 (1989)); and a method of adding an aqueous solution of a cobalt salt, an aqueous salt solution of a Fe(II), a strontium salt or a barium salt to a water dispersion of acicular iron oxide particles, and adding a magnesium compound in the process of the cobalt coating reaction (Japanese Patent Application Laid-Open (KOKAI) No. 4-168703 (1992)).
Magnetic iron oxide particles having a high coercive force, a large saturation magnetization and an excellent black chromaticity are now in the strongest demand. However, any of the magnetic iron oxide particles described in Japanese Patent Publication (KOKOKU) Nos. 4-25686 (1992), 62-50889 (1987) and 2-30563 (1990) and Japanese Patent Application Laid-Open (KOKAI) Nos. 1-184801 (1989) and 4-168703 (1992) cannot be said to be satisfactory.
In the magnetic iron oxide particles described in Japanese Patent Publication (KOKOKU) No. 4-25686 (1992), since a magnesium salt is added before the cobalt coating reaction, the Co-coated layer produced uniformly contains magnesium. It is thus impossible to form a two-layer structure composed of a lower layer composed of spinel ferrite containing cobalt and an upper layer composed of spinel ferrite containing cobalt and magnesium.
The magnetic iron oxide particles described in Japanese Patent Publication (KOKOKU) No. 4-25686 (1992) show an improved dispersibility in a magnetic tape obtained by dispersing the magnetic iron oxide particles in a binder.
The magnetic iron oxide particles described in Japanese Patent Publication (KOKOKU) Nos. 62-50889 (1987) and 2-30563 (1990) are obtained by coasting the surfaces of the magnetic iron oxide particles with a hydroxide or an oxide of magnesium. These particles show an improved dispersibility in a magnetic tape obtained by dispersing the magnetic iron oxide particles in a binder. In the magnetic iron oxide particles described in Japanese Patent Application Laid-Open (KOKAI) Nos. 1-184801 (1989), the magnetic stability with passage of time (magnetic temporal stability) is improved by adding an aqueous solution of a magnesium salt to an alkaline slurry of the cobalt-modified acicular maghemite particles and coating the surfaces of the particles with a hydroxide of magnesium. Acicular maghemite particles which contain no Fe.sup.2+ and, hence, which do not produce chemical and magnetic unstability with passage of time (chemical and magnetic temporal unstability) due to the existence of Fe.sup.2+, are used as the core particles of these magnetic iron oxide particles.
In the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-168703 (1992), a magnesium salt is added to a water dispersion containing barium and the like in the process of the cobalt coating reaction. When a magnesium salt is added, a barium salt or a strontium salt which has a larger ionization degree, already exists in the slurry. Therefore, since barium or strontium preferentially contributes to the formation of a spinel ferrite layer, it is difficult to form a spinel ferrite coating containing cobalt and magnesium (see Comparative Example 6 which will be described later).
Accordingly, the technical problem to be solved in the present invention is to produce magnetic iron oxide particles which have a high coercive force, a large saturation magnetization and an excellent black chromaticity, and which are magnetically and chemically stable with passage of time.
Fine magnetic particles have recently been used for enhancing the smoothness of the surface of a magnetic tape. On the other hand, if magnetic particles are fine, the specific surface area increases, so that a large amount of lubricant such as a fatty acid is absorbed to the particle surfaces and it is difficult to supply the lubricant to the surface of the magnetic tape, which may result in the deterioration of the running durability. This fact is described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 1-199316 (1989), ". . . A fatty acid or an ester of a fatty acid and an aliphatic alcohol is added to a magnetic layer as a lubricant so as to reduce the friction coefficient. However, in many cases of a magnetic recording medium using fine ferromagnetic particles, the lubricant cannot produce its desired effect . . . " Therefore, favorable magnetic particles are those which adsorb as little a fatty acid as possible, namely, magnetic particles having a small fatty acid adsorptivity.
As a result of studies undertaken by the present inventors so as to solve the above-described technical problems, it has been found that by heating and stirring a suspension obtained by adding an aqueous alkali hydroxide solution and an aqueous solution of a cobalt salt to an aqueous dispersion of acicular magnetite particles, or a suspension obtained by adding an aqueous alkali hydroxide solution, an aqueous solution of a cobalt salt and an aqueous solution of a ferrous salt to an aqueous dispersion of acicular magnetite particles, to form a Co-containing ferrite having a spinel structure (spinel ferrite containing cobalt) on the surface of the acicular magnetite particle; adding an aqueous solution of a magnesium salt to either of the suspensions in the process of a cobalt coating reaction for forming the Co-containing ferrite having a spinel structure (spinel ferrite containing cobalt) on the surface of the acicular magnetite particle; and continuing to heat and stir the resulting mixture so as to complete the cobalt coating reaction, thereby forming a Co-and Mg-containing ferrite having a spinel structure (spinel ferrite containing cobalt and magnesium) on the surface of the Co-containing ferrite coating layer which is formed on the surface of the acicular magnetite particle, the Co-coated acicular magnetite particles obtained have a high coercive force, a large saturation magnetization, an excellent black chromaticity and an excellent chemical and magnetic stability with passage of time (chemical and magnetic temporal stability). On the basis of this finding, the present invention has been achieved.